1. Field of the Invention
The present invention relates to a fixing apparatus used in a copying machine, a printer and the like for recording an image on a recording material in response to an original or image information.
2. Related Background Art
Conventionally, in image forming apparatuses, it is known to use a fixing apparatus having a thin wall fixing roller in order to reduce a rising time in power ON of a main body or in recovery after jam treatment. Further, in such a fixing apparatus, a thermistor (temperature detecting means) abuts against the surface of the fixing roller, and energization of a heater disposed within the fixing roller is controlled on the basis of a detected temperature from the thermistor.
However, when the thermistor is arranged within a sheet passing area of the fixing roller, contamination such as toner contamination is adhered to the surface of the thermistor during long term use to worsen temperature detecting accuracy, or an abutting trace of the thermistor is generated on the sheet passing area of the fixing roller to shorten the service life of the fixing roller.
To avoid this, it is considered that the thermistor is arranged in a non-sheet passing area or out of the sheet passing area of the fixing roller thereby to prevent the toner contamination on the thermistor and reduction in service life of the fixing roller.
In this case, however, since the temperature of the sheet passing area is not detected, temperature control of the sheet passing area of the fixing roller is not easy. Further, if heat capacity of the fixing roller is small, heat conductivity in the generatrix direction of the fixing roller becomes small.
In such a condition, energization of the heater is started by turning ON a power source of a main body of the image forming apparatus. Since the energization of the heater is continued until the detected temperature of the thermistor reaches a stand-by setting temperature (waiting condition is maintained in this time period), even if the heater is disenergized after the detected temperature of the thermistor reaches the stand-by setting temperature, the fixing roller is overshot due to its small heat capacity. Since the heat conductivity in the generatrix direction of the fixing roller is small, particularly the overshoot amount of a center portion of the fixing roller becomes great (refer to FIG. 3) in comparison with the small heat capacity.
It is preferable that the overshoot amount is reduced not to generate hot offset even when the copy (print) is started while the overshoot is being generated. Incidentally, since the fixing roller and pressing roller are rotated from a time t to a time 0 (FIG. 3), a temperature of the pressure roller is increased during this time period.
By the way, if there is no copy command after the waiting condition is released, the apparatus becomes a stand-by condition. In the stand-by condition, the energization of the heater is controlled so that the detected temperature of the thermistor is maintained to the setting temperature. Meanwhile, since the fixing roller and the pressing roller are not rotated to be kept stationary, the temperature of the pressing roller is gradually decreased (FIG. 3). Accordingly, the longer the stand-by time the smaller a heat accumulating amount of the pressing roller (particularly at both ends thereof having great heat discharging amount), thereby affect a bad influence upon improvement of fixing ability.
Although it is considered that the setting temperature in the stand-by condition is increased to solve the above problem, if doing so, the overshoot in the energization upon power ON becomes great.